1. Field of the Invention
This invention relates to viscous coupling fluids. More particularly, this invention relates to stable viscous coupling fluids containing combinations of polyorganosiloxanes with sulfur or with specified classes of sulfur containing organic compounds. These fluids are stable, as evidenced by the low degree of viscosity change and the absence of gelation during long periods of use.
2. Background Information
Axle joints that utilize the principles of viscous coupling have long been employed in differential mechanisms and drive shafts of four wheel drive vehicles because of the way viscous coupling improves driving stability and shock absorption on bad roads. Durability of viscous coupling is highly dependent on property of viscous coupling fluid.
Typical structures in which drive mechanisms employing viscous coupling are used consist of a housing and hub. The hub and the housing side will have a number of thin circular metal plates attached to it and will contain the highly viscous coupling fluid. A rotational differential is located between the hub and the plates on the housing side, and either torque is transferred or the rotational differential is controlled. Structures containing viscous coupling fluids are described in Japanese Laid-Open Patent Publication SHO 58-48779.
The viscous coupling is utilized for viscous resistance of viscous coupling fluids. In the case of automobiles, it is important that, so far as it is possible, a small device develops the required torque. This requirement results in the use of fluids of comparatively high viscosity. Additionally, the fluid used in viscous coupling should show little change in viscosity in relation to changes in temperature. Stability at high temperatures is important. Previously used silicone oils are typically dimethyl polyorganosiloxanes, which have a viscosity at 25.degree. C. of from 5,000-500,000 cSt.
The high temperatures produced by the shear force and the friction between the plates will often, over a period of time, finally cause the viscosity of the coupling fluid to rise until the fluid finally gels. This constitutes a problem in the use of the fluid. When the viscosity of the coupling fluid rises considerably and gels, the initial performance expectations for which the viscous coupling system was designed can no longer be met. A solution was sought for this problem.
On the other hand, when the coupling fluid is at a comparatively high temperature, heat decomposition causes a lowering of the viscosity with the result that once again the initial performance expectations for which the viscous coupling system was designed can no longer be met.
Various kinds of heat resistance additives for organopolysiloxane have been studied. The use of sulfur compounds for this purpose is described in U.S. Pat. No. 4,959,166 and the use of phosphorus compounds in Japanese patent publication Hei 2-91196.
However, the use sulfur or phosphorous compounds as additives does not have the desired effect. The present inventors observed good resistance to a substantial rise in viscosity and eventual gel formation due to polymerization or crosslinking using these additives, but the decrease in viscosity due to heat-induced decomposition cannot be prevented.
One objective of this invention is to provide a thermally stable fluid for use in viscous coupling systems. During use the viscosity of the present fluids neither increases, resulting ultimately in gel formation, nor decreases due to heat-induced decomposition.